1. Field of the Invention
This invention relates to an optical scanner to be suitably used for a copying machine, a printer or the like and also to a semiconductor laser array to be used for such an optical scanner.
2. Related Background Art
Known laser beam sources to be used for digital copying machines and printers comprise only a single semiconductor laser operating as light source as typically described in Japanese Patent Application Laid-Open No. 05-22530. However, in recent years, as a result of technological developments for a higher operating speed and a higher resolution level in the field of digital copying machines and printers, there is an increasing demand for light sources comprising a plurality of semiconductor lasers for emitting laser beams (so-called semiconductor laser arrays).
When using a semiconductor laser for an optical scanner, it is required to operate with an acceptable level of drooping property as described in Japanese Patent Application Laid-Open No. 1-205588. The drooping property is very important not only for those to be used for light sources comprising a single semiconductor laser but also for those to be used for semiconductor laser arrays (light sources comprising a plurality of semiconductor lasers to produce luminous spots).
Additionally, the cross talk property (that a luminous spot affects some other luminous spots) is also important to semiconductor laser arrays, although this property has nothing to do with light sources comprising a single semiconductor laser.
The cross talk property may be either thermal or electric. A thermal cross talk refers to a state where a luminous spot that is also emitting heat thermally adversely affects some other luminous spots to make them unable to emit light at a normal rate. On the other hand, an electric cross talk refers to a state where a luminous spot cannot be controlled for a normal rate of light emission if it is electrically connected to those luminous spots.
Thus, a semiconductor laser array that operates poorly in terms of cross talk also operates poorly in terms of drooping property. A conceivable measure that can be taken for reducing the level of thermal cross talk of a semiconductor laser array may be to make the semiconductor lasers of the semiconductor laser array structurally show a low threshold current and a high heat emitting efficiency.
As shown in Japanese Patent Application Laid-Open Nos. 10-051078 and 6-326210, buried heterojunction lasers (BH-LD) have already been proposed as a semiconductor laser having a structure for reducing the threshold current. The heat emitting efficiency of semiconductor lasers can be improved by means of soldering in junction down (J/D) arrangement.
However, buried heterojunction lasers show a luminous spot width smaller than conventional ones having a so-called planar structure so that the luminous spots of the buried heterojunction lasers of a semiconductor laser array are subjected to thermal stress in different directions in the vicinity thereof when the chip carrying the semiconductor lasers is soldered to a sub-mount. Then, in the case of a semiconductor laser array having a pair of luminous spots such as illustrated in FIG. 6, residual stress will be generated as running in direction A in the vicinity of luminous spot 3a, while residual stress will be generated as running in direction B in the vicinity of luminous spot 3b, to produce a difference in the angle of polarization between the two luminous spots so that there arise abnormal spot diameters and abnormal sub-scanning pitches when laser beams emitted from there pass through an optical part showing double refraction. Additionally, the junction down arrangement of soldering the chip to a sub-mount at positions near the luminous spots is easily subjected to stress at the time of soldering if compared with the junction up arrangement. For these reasons, scanning lenses are recently made of a plastic material that is available at low cost and used in a state where they show double refraction that arises at the time of molding, as described in Japanese Patent Application Laid-Open No. 10-048557.
While the intervals of the sub-scanning pitch may vary depending on the resolution, they are 21 μm if the resolution is 1,200 dpi and 42 μm if the resolution is 600 dpi. Because of the highly accurate intervals of the sub-scanning pitch, an error up to several μm is not permissible. In other words, if the error due to the difference in the angle of polarization is 5 μm with a resolution of 1,200 dpi, the intervals may be 16 μm or 26 μm depending on the position along the main-scanning direction. Then, a so-called banding phenomenon (of varying pitch where a wide interval and a narrow interval appear alternately) arises to make it difficult to provide a high resolution image.
Additionally, an error of about 5 μm occurs in the sub-scanning pitch if the difference in the angle of polarization is about 20 degrees, although it may depend on the extent of double refraction of the scanning lens.
As described above, with the prior art, since residual stress arises in different directions among the luminous spots of a semiconductor laser array when the chip carrying the semiconductor lasers is soldered to a sub-mount, the angle of polarization becomes variable as a function of the position of each luminous spot so that abnormal spot diameters and abnormal sub-scanning pitches are produced when laser beams pass through an optical part showing double refraction in an optical scanner.